The present invention relates to the depositing of a coating layer based on chromium on a support piece to provide a chromium coating with high hardness and resistance to wear, strain, fatigue and corrosion.
The board use made today of hard chromium electroplated coatings for improving the wear and corrosion resistance of mechanical parts, especially frictional parts, is known.
The many disadvantages and limitations of this type of plating or coating are also known. For example, the hardness from one thousand to eleven hundred Vickers (1000-1100 VH) on the average of the coating, while certainly higher than that of the majority of current mechanical construction materials, is nonetheless insufficient to resist severe abrasion wear effects. The coating has a tendency to form cracks, which for applications in a corrosive environment, makes it necessary to have recourse to either large deposit thicknesses (which is a definite handicap, given the slow deposition rate) or else protective sub-coatings, for example nickel, which accordingly increase the cost of the parts treated. There is a harmful effect on the fatigue resistance of plated or coated substrates by reason of the occlusion of hydrogen in the coating and of the formation of hydrates and oxides. The coating is too highly fragile, causing breaking of the coating in a case of strain of the substrate under the combined normal and tangential stresses. Finally, another problem is created by pollution by chromium (VI) contained in the effluents, requiring expensive neutralization installations.
Certainly processes are known which permit alleviating one or another of these disadvantages. For example, the technique of physical deposition in the vapor phase prevents pollution and leads to much less cracked deposits than those which can be achieved by the electroplating method; but the chromium thus obtained is generally low in hardness, from five hundred to six hundred Vickers (500 to 600 VH). Also, for example, coatings of chromium carbides or nitrides present a high hardness and good corrosion resistance, especially when they are achieved by the technique of physical deposition in the vapor phase, but their fragility makes them unsuited for resisting even a small strain on the substrate, etc.